We have measured the linear rheology of critically purified ring polyisoprenes, polystyrenes and polyethyleneoxides of different molar masses. The ratio of the zero-shear viscosities of linear polymer melts η0,linear to their ring counterparts η0,ring at isofrictional conditions is discussed as function of the number of entanglements Z. In the unentangled regime η0,linear/η0,ring is virtually constant, consistent with the earlier data, atomistic simulations, and the theoretical expectation η0,linear/η0,ring=2. In the entanglement regime, the Z-dependence of rings viscosity is much weaker than that of linear polymers, in qualitative agreement with predictions from scaling theory and simulations. The power-law extracted from the available experimental data in the rather limited range 1<Z<20, η0,linear/η0,ring ~ Z1.2±0.3, is weaker than the scaling prediction (η0,linear/η0,ring ~ Z1.6±0.3 ) and the simulations (η0,linear/η0,ring ~ Z2.0±0.3 ). Nevertheless, the present collection of state-of-the- art experimental data unambiguously demonstrates that rings exhibit a universal trend clearly departing from that of their linear counterparts, and hence it represents a major step toward resolving a 30-year old problem.
Keywords: entanglements; molecular weight; purification; ring polymers; viscosity.